This invention relates to mobile work stations such as for assembly, testing and operation of sensitive electronic devices. More particularly, the invention relates to a caster for a wheeled vehicle, such as a chair or a cart, which safely discharges static electricity and is suitable for use in a clean-room environment.
Many electronic devices can be easily damaged by discharges of static electricity, especially during manufacture and assembly. To prevent such damage, electrically conductive wheeled vehicles have been developed to provide a discharge path to ground potential in order to prevent the buildup of any static electricity. For example, an electrically conductive wheeled vehicle is disclosed in U.S. Pat. No. 4,747,011 for an anti-static chair, which is incorporated herein by this reference.
A problem associated with such wheeled vehicles is that it is difficult to provide sufficient electrical contact between the wheeled vehicle and a source of electrical ground, such as a floor surface. The electrical contact is typically provided through the casters of the wheeled vehicle.
A conventional way of providing such contact is to make the wheels of the casters out of conductive material, as is disclosed in U.S. Pat. No. 4,747,011. However, with such a construction of the casters, the dust resulting from wear of the casters is conductive. Accumulations of conductive dust on a floor surface of a clean-room environment are undesirable because electrically charged dust particles can cause damage to the electrical devices in the work station environment. A further problem occurs when the caster wheels are made of carbon. The carbon tends to leave black smudges on the floor surface.
Other means of providing contact between one or more casters of the wheeled vehicle and the floor surface have been developed. One such means includes a conductive sphere attached to one end of a conductive spring, with the other end of the spring mounted to a frame member of the caster. A wire segment is attached to the spring at one end of the segment. Another end of the wire segment is attached to the body of the caster by a metal staple which is in electrical contact with the conductive mounting of the caster. During operation of the wheeled vehicle, the spring pushes the sphere against the floor surface thus providing the necessary contact with a source of electrical ground.
A disadvantage of the spring/sphere conducting means described above is that the sphere can become lodged in an opening in the floor surface, such as a crack or a separation between two slabs of concrete. The sphere can also be snagged by a carpet loop. The spring then tends to become stretched beyond its capacity to recoil. The distended spring is unsightly and no longer pushes the sphere against the floor surface and effective electrical contact is thereby diminished or lost. Moreover, the distended spring can become entangled with the caster wheels.
Another disadvantage of the spring/sphere conducting means is that it is comparatively expensive and complex to make. A relatively complex frame member is required for mounting the spring and wire segment on the caster. In addition, the spring must be tightly wound to provide adequate recoiling capacity and must be stretched slightly to provide sufficient contact with the floor. Moreover, the sphere is difficult to attach to the spring and therefore must be constructed with a special attachment piece.
Accordingly, there is a need for a conducting caster for use in connection with a conducting wheeled vehicle that does not easily become distended or broken and is simple and inexpensive to make.